What causes a SPN 421 code?

SPN 421 can be triggered by several issues — see the verified probable causes and step-by-step diagnostics shared by mechanics and owners on OBDIY.

Can I drive with a SPN 421 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 421 active.

How do I fix SPN 421?

Follow the step-by-step diagnostic on this page, then read the verified fixes contributed by other Heavy Truck owners — the answer marked "Verified Cause" worked for the original poster.

OBDIY

J1939 SPN

SPN 421

J1939 SPN 421 — Fuel/Air parameter

What it means

SPN 421 indicates a fault in the Engine Cold Start Fuel Heater circuit or a rationality mismatch in the fuel/air temperature parameters used for cold start optimization.